Re: [PATCH v2] kallsyms: strip LTO-only suffixes from promoted global functions

[Yonghong Song <yhs@meta.com>]

On 7/7/23 5:23 PM, Yonghong Song wrote:
> 
> 
> On 7/7/23 3:27 PM, Nick Desaulniers wrote:
>> On Wed, Jun 28, 2023 at 11:19 AM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> Commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static 
>>> functions")
>>> stripped all function/variable suffixes started with '.' regardless
>>> of whether those suffixes are generated at LTO mode or not. In fact,
>>> as far as I know, in LTO mode, when a static function/variable is
>>> promoted to the global scope, '.llvm.<...>' suffix is added.
>>>
>>> The existing mechanism breaks live patch for a LTO kernel even if
>>> no <symbol>.llvm.<...> symbols are involved. For example, for the 
>>> following
>>> kernel symbols:
>>>    $ grep bpf_verifier_vlog /proc/kallsyms
>>>    ffffffff81549f60 t bpf_verifier_vlog
>>>    ffffffff8268b430 d bpf_verifier_vlog._entry
>>>    ffffffff8282a958 d bpf_verifier_vlog._entry_ptr
>>>    ffffffff82e12a1f d bpf_verifier_vlog.__already_done
>>> 'bpf_verifier_vlog' is a static function. '_entry', '_entry_ptr' and
>>> '__already_done' are static variables used inside 'bpf_verifier_vlog',
>>> so llvm promotes them to file-level static with prefix 
>>> 'bpf_verifier_vlog.'.
>>> Note that the func-level to file-level static function promotion also
>>> happens without LTO.
>>>
>>> Given a symbol name 'bpf_verifier_vlog', with LTO kernel, current 
>>> mechanism will
>>> return 4 symbols to live patch subsystem which current live patching
>>> subsystem cannot handle it. With non-LTO kernel, only one symbol
>>> is returned.
>>>
>>> In [1], we have a lengthy discussion, the suggestion is to separate two
>>> cases:
>>>    (1). new symbols with suffix which are generated regardless of 
>>> whether
>>>         LTO is enabled or not, and
>>>    (2). new symbols with suffix generated only when LTO is enabled.
>>>
>>> The cleanup_symbol_name() should only remove suffixes for case (2).
>>> Case (1) should not be changed so it can work uniformly with or 
>>> without LTO.
>>>
>>> This patch removed LTO-only suffix '.llvm.<...>' so live patching and
>>> tracing should work the same way for non-LTO kernel.
>>> The cleanup_symbol_name() in scripts/kallsyms.c is also changed to 
>>> have the same
>>> filtering pattern so both kernel and kallsyms tool have the same
>>> expectation on the order of symbols.
>>>
>>>   [1] 
>>> https://lore.kernel.org/live-patching/20230615170048.2382735-1-song@kernel.org/T/#u
>>>
>>> Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static 
>>> functions")
>>> Reported-by: Song Liu <song@kernel.org>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>
>> Thanks for the patch and improving live patch with LTO.  Looking back
>> at the internal report that resulted in
>> commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static 
>> functions")
>> your version was what I originally had.  I did not leave a comment as
>> to why I changed it when I sent it 2 years ago, and no longer recall
>> the reason.
>>
>>
>> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> Nick, thanks for the review. I am not sure how this patch could
> be merged into the mainline. I checked your patch
>     6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> and it looks like the patch was merged by Kees Cook.
> 
> I added Kees in the 'To' list. Kees, could you help merge
> this patch if you are okay with the change?

Looks like Kees might be on vacation. Nick, Petr, do you
have any suggestions who could take this patch? Thanks!

> 
> Thanks!
> 
> Yonghong
> 
>>
>>> ---
>>>   kernel/kallsyms.c  | 5 ++---
>>>   scripts/kallsyms.c | 6 +++---
>>>   2 files changed, 5 insertions(+), 6 deletions(-)
>>>
>>> Changelogs:
>>>    v1 -> v2:
>>>      . add 'Reported-by: Song Liu <song@kernel.org>'
>>>      . also fix in scripts/kallsyms.c.
>>>
>>> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
>>> index 77747391f49b..4874508bb950 100644
>>> --- a/kernel/kallsyms.c
>>> +++ b/kernel/kallsyms.c
>>> @@ -174,11 +174,10 @@ static bool cleanup_symbol_name(char *s)
>>>           * LLVM appends various suffixes for local functions and 
>>> variables that
>>>           * must be promoted to global scope as part of LTO.  This 
>>> can break
>>>           * hooking of static functions with kprobes. '.' is not a valid
>>> -        * character in an identifier in C. Suffixes observed:
>>> +        * character in an identifier in C. Suffixes only in LLVM LTO 
>>> observed:
>>>           * - foo.llvm.[0-9a-f]+
>>> -        * - foo.[0-9a-f]+
>>>           */
>>> -       res = strchr(s, '.');
>>> +       res = strstr(s, ".llvm.");
>>>          if (res) {
>>>                  *res = '\0';
>>>                  return true;
>>> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
>>> index 0d2db41177b2..13af6d0ff845 100644
>>> --- a/scripts/kallsyms.c
>>> +++ b/scripts/kallsyms.c
>>> @@ -346,10 +346,10 @@ static void cleanup_symbol_name(char *s)
>>>           * ASCII[_]   = 5f
>>>           * ASCII[a-z] = 61,7a
>>>           *
>>> -        * As above, replacing '.' with '\0' does not affect the main 
>>> sorting,
>>> -        * but it helps us with subsorting.
>>> +        * As above, replacing the first '.' in ".llvm." with '\0' 
>>> does not
>>> +        * affect the main sorting, but it helps us with subsorting.
>>>           */
>>> -       p = strchr(s, '.');
>>> +       p = strstr(s, ".llvm.");
>>>          if (p)
>>>                  *p = '\0';
>>>   }
>>> -- 
>>> 2.34.1
>>>
>>
>>